1. Field of the Invention
The present invention relates generally to methods and apparatuses for forming images, and methods for producing printed products.
2. Description of the Related Art
Digital electrophotographic printers have come into wide use in the field of production printing in recent years. Accordingly, there is a demand for an increase in image quality and reliability of digital electrophotographic printers. In particular, there is a strong demand that digital printers be increased in uniformity of gray levels.
A digital electrophotographic printer typically includes an image processing unit which performs image processing on image data to correct and thereby increase image quality. An image processing unit may perform image processing on multi-bit image data of high resolution of 1,200 dots per inch (dpi) or 2,400 dpi, for example.
Such a digital electrophotographic printer typically includes: a photosensitive drum of which photosensitive surface functions as a surface to be scanned; a light source which emits laser light; a polygon mirror which deflects the laser light emitted from the light source; and a scanning optical system which causes the laser light deflected by the polygon mirror to impinge on the surface (the surface to be scanned) of the photosensitive drum. The digital electrophotographic printer may form an electrostatic latent image representing image data on the photosensitive drum by modulating a light beam emitted from the light source according to the image data, illuminating the surface to be scanned with the light beam from the light source, and scanning the light beam across the surface to be scanned.
A digital electrophotographic printer configured as described above typically employs a device, such as a laser diode array (LDA) or a vertical-cavity surface-emitting laser (VCSEL), having a plurality of light emitters as the light source. Employment of such a device allows the digital electrophotographic printer to form an electrostatic latent image of a resolution of, for example, 2,400 dpi or 4,800 dpi, higher than that of 1,200-dpi image data.
There can be a situation in which an image processing unit of a digital printer applies, as digital half-toning, parallel-line dithering which replaces each uniform-gray-level region (put another way, region of a same pixel value) of image data with a parallel-line pattern. Each width of lines constituting the parallel-line pattern varies depending on the pixel value, so that each gray level is expressed as an area percentage.
For example, Japanese Laid-open Patent Application No. 2004-282344 discloses a technique, in which an image processing unit suppresses image noise without changing screen ruling by generating a binary parallel-line pattern by digital half-toning, increasing resolution of the parallel-line pattern, and thereafter performing interpolation and smoothing. Japanese Patent No. 4026271 discloses a technique which increases image quality by increasing resolution by dividing pixels of an image into subpixels, and smoothing edge portions of the image.
Meanwhile, when a parallel-line pattern is high in line density, an electrostatic latent image of the pattern formed on a photosensitive drum can have a region where electric field intensity is weak due to an influence exerted from an adjacent line or the like. This is undesirable because, in such a region, toner sticks to the photosensitive drum less reliably.
Edges of lines of a parallel-line pattern representing low-resolution image data have a shape (stepped profile) assuming a large-step staircase. For this reason, when a highly-dense parallel-line pattern of low-resolution image data is formed on a photosensitive drum, a weak electric-field region can be produced particularly at a stepped profile portion where lines of the pattern are close to each other. In the weak electric-field region, toner can undesirably unevenly stick to the photosensitive drum or be scattered between the lines. Accordingly, it has been difficult for digital printers to form an image of uniform gray levels from low-resolution image data to which parallel-line dithering is applied.
Under the circumstances described above, there is a need for methods and apparatuses for forming images, and methods for producing printed products with parallel-line patterns with uniform gray levels.
It is an object of the present invention to at least partially solve the problem in the conventional technology.